1. Field of the Invention
The present invention relates to an arrangement for the electrogalvanic metal coating of strips which travel through an acid electrolyte enriched with metal. The arrangement includes at least one insoluble anode arranged parallel to the strip, wherein the current flows to the strip switched as the cathode, and wherein metal is deposited from the electrolyte on the surface of the strip.
2. Description of the Related Art
Cold-rolled strip of normal carbon steel must be provided with a protective layer in order to prevent corrosion or at least significantly delay the corrosion. The type of protective layer depends on the intended use and the economical feasibility.
One method known in the art is galvanizing. When the strip is galvanized, the corrosion protection is achieved by a metal coating which is applied electrolytically.
Plants for applying such zinc layers on one side or both sides of the strip in thicknesses of about 2.5 to 15 micrometers are known in the art. The anodes are arranged parallel to the strip at as small a distance as possible of between 5 and 30 mm. The space between each anode and the strip is filled with an acid electrolyte which is enriched with metal, i.e., zinc. During coating, the current flows from the anodes to the strip which is switched as the cathode and the zinc is deposited on the surface of the strip.
In these conventional arrangements, there are problems when coating on one side as well as when coating on both sides. The current flux density increases toward the edges of the strip. Consequently, an extremely high current density occurs at the strip edges which leads to an increased depositing of zinc. Therefore, the thickness of the zinc layer in the edge region of the strip is about 2 to 3 times greater than in the middle of the strip.
Aside from the wasted metal and energy, this results in problems when coiling the strip and in the subsequent processing steps. For this reason, the edges of the strips must be trimmed over a great width prior to coiling which leads to a significant material loss as well as additional work.
If such an arrangement is to be used to coat the strip only on one side, there are additional problems. If the anode of the strip which is not be coated is completely removed or is replaced by a dummy anode, for example, a plastic plate, not only the edges of the side to be coated are galvanized, but because the current flows around the edge, the edges of the side not be coated are also galvanized.
If the anode on the side not be coated is merely electrically switched off, there is the additional problem that metal is deposited also on the strip side which is not to be coated. The reason for this is that current flows from the anodes which are wider as compared to the strip outside of the strip area through the electrolyte onto the switched-off anode and, thus, this anode is under voltage relative to the strip.
In order to solve these problems, so-called edge masks have become known. These masks are in the form of electrically insulating plates or foils and prevent the current from flowing between the two anodes next to the strip.
The strip edges engage in U-shaped sections arranged at the end faces of the electrically insulating plates. The degree of edge galvanization depends on the insertion depth of the strip edges into the U-shaped sections. Accordingly, it is necessary that the U-shaped sections always very exactly follow the strip travel. This requires a strip edge position measurement and complicated edge mask drives with complicated measuring and regulating technology.
Another disadvantage of the edge masks is the fact that they are susceptible to trouble. For example, when the strip edges are not smooth or when width variations of the strip occur suddenly, the edge masks may be damaged. Expensive idle times and repairs are the consequence.
Finally, the edge masks require a minimum distance between the anodes in order to be able to construct the edge masks with sufficient stability.
In addition, the edge masks do not solve the problem that the coating thickness over the width of the strip is a direct reflection of the transverse section of the strip. For example, if the strip has a transverse arc or other non-planarities or inclined positions between the anodes, this results in a non-uniform coating thickness. In order to prevent this undesired effect, the prior art provides for expensive stretching and straightening plants arranged upstream of the coating processes.